Builder.buildDir should respect .dockerignore

[pipex]

Although the library has an option to create a build stream from an archive, Builder.buildDir is the most handy option building an image from a directory. However, if the directory has build files (e.g. node_modules, library files, etc), these will end up in the tar, delaying the build start. Having the method read from .dockerignore to skip files for the build, might be a way to solve the issue.